It makes sense that what we’re exposed to can affect our health, including our fertility. And the latest research shows exactly how much.

Reporting in the journal Environmental Health Perspectives, researchers say that pollutants such as perchlorinated biphenyls (PCBs), industrial compounds and pesticides that are no longer manufactured but remain in older products can still decrease couples’ ability to have children by up to 29%.

Scientists have long known that farm and factory workers exposed to certain chemicals at high levels experience declines in fertility. But whether the same is true for those exposed to ubiquitous hormone-disrupting chemicals at low levels, frequently without our knowledge, isn’t clear yet.

So scientists at the National Institutes of Health created the Longitudinal Investigation of Fertility and the Environment (LIFE), the most comprehensive look yet at various environmental pollutants and their potential effect on pregnancy rates, to capture the effects of the compounds not just on female reproductive health but on both male and female fertility. The trial followed 500 couples who stopped using contraception for a period of either 12 months or until they got pregnant, whichever came first. Researchers measured their blood for the presence of 63 organic pollutants such as (PCBs) found in oil-based paint, electrical parts and adhesives until they were banned in 1979, and pesticides that fail to degrade in the environment but are absorbed by livestock and then by people consuming fatty fish, meats and dairy.

Each couple was given a kit to help them monitor fertility-related hormones to optimize their chances for pregnancy, and the couples kept track of what they ate and other lifestyle behaviors in a daily journal. At the end of the study period, researchers analyzed blood levels of the designated environmental chemicals and the length of time each couple took to get pregnant and found that for each unit increase in blood concentration of 12 pollutants was associated with anywhere from a 17% to 29% decrease in odds of achieving pregnancy. The relationship remained significant after adjusting for the effects of age, body mass index and smoking, all of which are known to affect fertility.

“This is a very special study and there haven’t really been others like it,” says Shanna Swan, a professor at Mt. Sinai School of Medicine, who conducts research in fertility but was not involved in the LIFE study. “It is extremely well done and it demonstrates that chemicals do affect time to pregnancy. I think it’s going to be a game changer.”

The American Chemistry Council, a trade organization representing industry manufacturers, says the study may not be considering all of the factors that affect fertility. “Many factors can affect when or if a pregnancy occurs and, while this study attempted to address some of those outside issues, not all were taken into account,” the council said in a written response to TIME about the findings.

One of the most striking findings of the study included the stronger associations between exposure to chemicals and fertility among men. For women, blood levels of one chemical previously used to make stain resistant products and three PCBs were associated with 18% to 21% lower odds of getting pregnant. For men, higher blood concentrations of eight chemicals, including one pesticide and seven PCBs, were associated with a 17% to 29% percent reduction in the odds that his partner would get pregnant. In fact, the association for some of these chemicals is as strong as the association between reduced fertility and age, study authors say.

“When we talk about time to pregnancy we always say ‘Well, Aunt Jane can’t get pregnant or Aunt Sara took forever,'” says Germaine Buck Louis, the paper’s lead author and an epidemiologist at the NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Health. “It is always described with a female name. Well, the men do matter.”

And it wasn’t just the public that perpetuated this notion. “For a long time in science, the feeling was that Dads didn’t contribute much other than the rigid unfolding of the genome,” Louis says.

She says most studies that have tried to look at chemical exposures and time-to-pregnancy in the past have studied the exposure levels of women who are already pregnant. This design not only excludes the women who might provide the most revealing information about how these chemicals may affect fertility, because they never get pregnant, but also leaves out the men. When Buck wrote grant proposals for time-to-pregnancy studies in the past that included men, reviewers rejected the idea because they didn’t believe that men would keep diaries or be interested in participating.

As a result, much of the research on male fertility and chemical exposures to date has focused on semen quality and male hormone levels. Still, says Harvard University’s Dr. Russ Hauser, an epidemiologist who has conducted some of these studies, there are precious few of these studies and the results are mixed. “I think the biggest shortcoming is the lack of data rather than the lack of data showing an association,” he says.

That gender gap in data is being addressed, but slowly. While studies have documented the trend toward early puberty in girls, and the potential environmental factors that may be contributing to the shift, only recently have scientists turned their attention to a similar trend among boys.

But as the latest results show, understanding the factors that may lower fertility among not just women but men as well may be critical to finding more effective ways of helping infertile couples to start families.